In the past, fibers have been used for their electrical properties, and fibers which possess a degree of electrical conductivity have been incorporated into articles to increase the conductivity of the article and to provide a measure of electrostatic discharge (ESD) protection to the article. Types of fibers utilized for their electrical conductivity include naturally occurring fibers, such as wool, which provide a measure of electrical conductivity due to the fact that a certain amount of moisture is normally found on the fiber's outside surface. Moisture associated with the fiber's outside surface can provide a conductive pathway, thereby permitting static electric charges present on the outside surface of the fiber to dissipate.
Man-made fibers based upon commonly produced polymeric materials used in the production of fibers such as polyamides or polyesters have been used to produce fibers which possess a degree of electrical conductivity. These man-made fibers may be treated on their outside surfaces with a conductive agent to increase the finishes which are applied to the outside surface of the fiber. Durability of antistatic finishes are usually less than the fiber on which the antistatic finishes are placed. Fibers which rely on such finishes for electrical conductivity can gradually lose their antistatic finishes while in use or through a cleansing process and become less electrically conductive overtime.
Conductive agents may also be in the form of a coating of a metal or carbon black placed on the outside surface of a fiber. The durability of the coating of metal or carbon black is dependent on the ability of these materials to bond and remain bonded to the outside surface of the fiber. If the coating is less flexible than the fiber on which it is placed, the coating may crack producing discontinuities in a conductive pathway provided by the coating.
Conductive agents have been incorporated into man-made fibers to provide a permanently conductive fiber. Conductive agents that have been incorporated into man-made fibers include antistatic finishes, carbon blacks and powdered metals. The conductive agents may be distributed throughout the man-made fiber or may be contained within a conductive core or strip. The electrical properties of these fibers usually remain for the life of the fiber. However, the polymeric materials used to produce these fibers, such as polyamides or polyesters have utility over a relatively narrow range of temperatures and chemical and environmental conditions.
Polytetrafluoroethylene (PTFE) exhibits utility over a relatively wide range of temperatures and chemical and environmental conditions. PTFE is usable over a temperature range from as high as 260.degree. C. to as low as near -2730.degree. C. PTFE is also highly resistant to attack from many harsh chemical reagents. However, PTFE does not possess exceptional strength. A form of PTFE, expanded porous polytetrafluoroethylene (EPTFE) as produced by the method taught in U.S. Pat. No. 3,953,566 to Gore, exhibits higher strength than PTFE. EPTFE is an excellent dielectric material and has been used as an insulative layer on wire and cable applications.
ePTFE in film form has been filled with various fillers as taught in U.S. Pat. Nos. 4,187,390 to Gore and 4,985,296 to Mortimer, Jr. Conductive fillers are taught as well in Gore and Mortimer, Jr., however, the filled EPTFE articles taught are in film form and not in fiber form.
The present invention is directed to EPTFE fibers which are filled with an amount of conductive filler thereby imparting a degree of electrical conductivity to the fiber.